The present invention is related to machine vision vehicle wheel alignment systems configured to view an optical target associated with a vehicle wheel to determine vehicle wheel alignment angles, and in particular, to a machine vision vehicle wheel alignment system which includes an optical projection system adapted to project a target pattern onto a vehicle wheel to aid in the determination of vehicle wheel alignment angles.
Conventional machine vision wheel alignment systems such as shown in U.S. Pat. No. 5,535,522 to Jackson and in U.S. Pat. No. 5,675,515 to January use predefined target structures which are physically mounted to the wheels of a vehicle. The purpose of these target structures is to provide a very accurately known fixed pattern on a stable surface so that the physical target position, relative to the observing camera, can be determined by analysis of the acquired images. Rolling of the vehicle with the physical targets secured to the vehicle wheels, allows the vehicle wheel alignment system to calculate the target position(s) with respect to the wheel(s) axis of rotation. Subsequent coordinate transformations establish the individual wheel positions in a common coordinate system, from which vehicle alignment angles may be determined.
Those of ordinary skill in the art of vehicle wheel alignment will recognize that the accuracy of the alignment angle measurements determined from images of physical target structures depends upon the precision of the target image analysis and proper compensation for the physical target location with respect to the axis of rotation of the associated vehicle wheels. In normal use, physical target patterns and structures may become corrupted by greasy handprints and damage from impact or abrasion with sharp objects. Image analysis software utilized by traditional machine-vision vehicle wheel alignment systems has some tolerance for corrupted target patterns, and will provide accurate measurements so long as these thresholds are not exceeded. When an image of a physical target can no longer be analyzed by the vehicle wheel alignment system, a technician must stop the alignment process, investigate the cause, and take corrective action. Most often only a thorough cleaning is required, but in severe cases the physical target must be replaced. Regardless of the cause, time is lost and efficiency decreased.
Another disadvantage to using physical target structures is apparent when the target structures are required to be mounted on the vehicle wheels, and lies in the use of a wheel adapter to secure the target to the wheel. Wheel adapters can be clumsy and time consuming to apply. If the wheel adapter is not properly secured to the wheel, or the target structure is not properly secured to the wheel adapter, the target structure can fall off or shift position during a compensation procedure resulting in lost time or erroneous alignment angle measurements. The technician may not know or suspect one of the alignment angles was improperly measured unless the error was very large and would likely attempt to correct the “bad” angle as a normal part of the alignment service, unwittingly introducing misalignment into the vehicle. These conditions may or may not be discovered during a subsequent road test of the vehicle following the alignment procedures.
Generally, machine vision vehicle wheel alignment systems requiring physical target structures and wheel adapters are not forgiving to corruption of the target patterns or adapter mounting errors. It takes time and money to properly maintain these items and they are expensive to replace if damaged beyond repair.
Accordingly, a machine vision vehicle wheel alignment system which does not require physical target structures or wheel adapters to be mounted to vehicle wheels would be desirable. Target cleaning and maintenance requirements could be eliminated, and problems with wheel adapter mounting errors would not be an issue. By eliminating physical target structures, the time required to initially secure wheel adapters to the vehicle wheels is no longer part of the alignment process, and shorter alignment procedure cycles can be achieved.
Additional advantages may be obtained by providing a machine vision vehicle wheel alignment system which does not rely upon a complex arrangement of mirrors, cameras, or lasers, and which does not require any physical markings or adhesive targets to be placed on a vehicle wheel assembly prior to beginning a vehicle wheel alignment procedure.